docs: remove private key export examples

[charliebot87]

Summary

• remove CLI examples that put PVT_K1 private keys directly on the command line
• remove the proton key:get export example from the general CLI reference
• replace troubleshooting guidance with interactive keychain import only

Verification

• ./scripts/validate-skill.sh
• grep check for proton key:get and direct PVT_K1 key-management examples